You would need to use something like this CSS code to change that menu colour:

/* Changes background color */
menupopup, popup,
popup > menu > menupopup,
menupopup > menu > menupopup {
    -moz-appearance: none !important;
    background: black !important;
}

/* Changes the color of the separator lines */
menupopup menuseparator {
    -moz-appearance: none !important;
    background: white  !important;
    padding: 0 !important;
    border-color: transparent !important;
}

/* Changes the text color */
menupopup menu,
menuitem {
    -moz-appearance: none !important;
    color: white !important;
}

/* Changes the hover colors */
menupopup menu:hover,
menuitem:hover {
    -moz-appearance: none !important;
    color: black !important;
    background-color: skyblue !important;
}

/* Changes the arrow of the submenu to light color
    Remove this if you want to keep the arrow a dark color */
.menu-right{
    filter: invert(100%) !important;
}

Hope this helps.

If you need more CSS tweaks like this, I'd recommend visiting the FirefoxCSS Reddit. It's an entire community of people who are able to help with this kind of thing.

Thanks for your help, I tried the first one, as I think that is what I am looking for. Unfortunately it does not work.

Unfortunately, the FirefoxCSS subreddit is probably your best bet on this issue. There are tons of people on there who will likely be able to help you find/create the perfect CSS code to fix your issue.